This invention relates to a converter for a cable television system or the like, and more particularly to a baseband converter for descrambling sync suppressed video television signals.
Cable systems typically provide basic services for a fixed monthly fee, and one or more premium channels at an additional cost. In order to prevent subscribers who have not paid for a premium channel from viewing the premium channel, such channels are usually scrambled or encrypted. Those subscribers who are authorized to receive the premium channel are provided with a descrambler to enable the channels to be viewed. In response to the various services provided by cable television operators, television manufacturers are offering models which tune both broadcast and cable only channels. Televisions, video recorders, and other video appliances adapted to receive cable channels are often referred to as "cable ready" or "cable compatible".
In present day systems, the term "cable ready" is somewhat of a misnomer. Even though the cable ready video appliance can receive cable channels without a separate tuner, the video appliance is not able to descramble premium channels. A separate descrambler box is still required to descramble the channel signals before they are input to the user's video appliance. The separate descrambler box, also referred to as a "converter/decoder" or simply "converter", tunes the full range of cable channels and decodes or unscrambles premium channels as necessary, providing an output to the subscriber's television receiver on a single broadcast channel, typically channel 3 or channel 4. A subscriber is thus unable to make full use of remote tuning and other features of his cable ready appliance. Where the converter from the cable company and the user's own video appliance both employ remote control units, confusion often arises as to which remote control is needed to obtain a desired operation. Further, unnecessary expense results from the duplication of tuning, intermediate frequency ("IF") and remote control systems in both the converter and the subscriber's video appliance.
Television signals broadcast through the air or over a cable are transmitted at radio frequencies ("RF"). In a conventional converter, television set, or other video appliance, a tuner is provided to tune to a single channel out of a plurality of channels traveling down a cable system or through the airwaves. Then, the tuned channel is mixed with a sinewave and changed to a different, intermediate frequency. This intermediate frequency signal is demodulated to provide a "baseband" television signal at a frequency well below the RF band.
One solution to the problems noted above with respect to cable ready video appliances is to provide a baseband interface on the video appliance (e.g., a cable subscriber's television receiver) to accommodate a converter designed to be compatible with the interface. Such an interface enables a channel to be selected via the tuner in the subscriber's video appliance, instead of by a separate tuner in the converter. The baseband signal produced by the subscriber's video appliance is then accessed and used as the input to the converter. The converter descrambles the baseband signal, and returns it to the display and audio section of the subscriber's video appliance. This scheme permits the cable operator to reduce the cost of equipment which is normally provided to the subscriber. At the same time, since the subscriber is using the tuner and other controls on his own television set or other video appliance, he can make full use of the features provided in his video appliance, and contend only with the remote control provided with his appliance.
A standard for such an interface has been published by the Consumer Electronics Group, Electronic Industries Association of Washington, D.C., U.S.A., as EIA Interim Standard IS-15, entitled, Standard Baseband (Audio/Video) Interface Between NTSC Television Receiving Devices and Peripheral Devices. This standard is incorporated herein by reference.
A common technique for scrambling premium channels provided over a cable television system is known as sync suppression. In this scheme, the horizontal and/or vertical synchronization pulses normally found in the video portion of the television signal are attenuated so that a television receiver cannot lock onto them, preventing the proper display of the video signal. In order to render the signal useable, it must be "descrambled" by restoring the synchronization pulses to a normal, unsuppressed level.
It would be advantageous to provide a converter for use with a television or other video appliance using the IS-15 standard. Such a converter would have to descramble a baseband video signal. It would be further advantageous to provide such a converter that descrambles sync suppressed video television signals at baseband frequencies. The present invention relates to such a converter.